[!297] makes jit type field matrix for generic mixin

Merge branch 'feature/copy-jit-into-fm' into 'master'

ref:extensions/dune-codegen see discussion in [!293].

See merge request [extensions/dune-codegen!297]

  [!293]: gitlab.dune-project.org/NoneNone/merge_requests/293
  [extensions/dune-codegen!297]: gitlab.dune-project.org/extensions/dune-codegen/merge_requests/297